An amazing new study claims that there are vast tunnels that lie underneath the lunar surface, an idea first theorized decades ago but now has major new evidence to support it. And scientists say it makes sense, because lava from ancient volcanoes could have bored holes through the moon’s surface eons ago, as it volcanoes have done in Hawaii.

Japanese researchers say they’ve found proof that these tunnels exist using the Kaguya orbiter, which is the biggest lunar mission since the Apollo program. It was launched back in 2007 and uses fine-tuned instruments to unlock the mysteries of our lunar satellite.

The finding stems from when Kaguya flew 60 miles above the Marius Hills in 2009 and snapped a picture of a big hole, not a terrible unusual site. But when the NASA Lunar Reconnaissance Orbiter took a follow-up shot, they found that the hole appeared to extend eastward and westward under a ceiling, like the mouth of a tunnel.

“Lunar lava tubes are sub-surface tunnels on the Moon that are believed to have formed during basaltic lava flows,” according to a Wikipedia excerpt. “When the surface of a lava tube cools, it forms a hardened lid that contains the ongoing lava flow beneath the surface in a conduit-shaped passage. Once the flow of lava diminishes, the tunnel may become drained, forming a hollow void. Lunar lava tubes are formed on surfaces that have a slope that ranges in angle from 0.4–6.5°. Lunar lava tubes may be as wide as 500 metres (1,600 ft) before they become unstable against gravitational collapse. However, stable tubes may still be disrupted by seismic events or meteoroid bombardment.”